FIG. 8 shows an example of conventional wafer transfer apparatuses for taking out a wafer from a wafer cassette by means of a transfer arm. More specifically, a wafer cassette 4 storing a number of wafers 3 is mounted on a cassette base 2 movable up and down by an elevator mechanism portion 1, such that a wafer-inlet/outlet side faces a transfer arm 5 having an attracting portion 6. The transfer arm 5 has a general structure movable forward and backward relative to the wafer cassette 4.
An operation of the conventional wafer transfer apparatus will be described in brief. First, the cassette base 2 is moved down by the elevator mechanism portion 1, and the number of detected wafers is counted by sensors (not shown), thereby detecting a wafer 3 to be transferred. After the sensor detects the wafer 3, the cassette base 2 is moved up by a predetermined amount, and the transfer arm 5 is moved toward the wafer cassette 4 and inserted under the wafer 3 to be transferred. After the transfer arm 5 is inserted, the cassette base 2 is moved down by a predetermined amount by means of the elevator mechanism portion 1, so that the wafer to be transferred is placed on the transfer arm 5. Then, the wafer 3 to be transferred is attracted by the attracting portion 6, the transfer arm 5 is drawn out of the wafer cassette 4, and the wafer 3 to be transferred is extracted and transferred to a next step, such as an inspecting step.
In the wafer transfer apparatus described above, a plurality of slot grooves 7, for holding the wafers 3 horizontally, are formed in both side walls in the wafer cassette 4 for storing the wafers 3. A view encircled in a right portion of FIG. 8 is an enlarged view showing slot grooves 7 in the right side wall of the wafer cassette 4 viewed from the wafer 3-inlet/outlet side. As shown in the drawing, the cross section of a slot groove is a tapered trapezoid, so that the area in which a circumferential edge portion of the wafer 3 contacts to the groove can be as small as possible, to reduce the friction between the groove 7 and the wafer 3, keep the wafer 3 horizontally and make it easy to take in and out the wafer 3.
FIG. 9 is a cross sectional view of the wafer cassette 4 storing the wafers 3, taken along a line between slots and viewed from a position above the apparatus. The top side of FIG. 9 corresponds to the wafer-inlet/outlet side. The dot line represents a walls connecting member provided on a bottom portion of the cassette 4.
As shown in FIG. 9, the cassette 4 has a trapezoidal shape, the width of which decreases toward the back side, for the purpose of preventing the wafer 3 from dropping down, when the disk-shaped wafer 3 is stored in each slot groove 7. More specifically, a back-side opening 8, which opens in the back side of the wafer cassette 4 (the bottom side of FIG. 9), is narrower than the diameter of the wafer 3, so that the wafer 3 is brought into contact with back-around portions 701. Thus, the wafer is prevented from dropping down from the back side of the wafer cassette 4.
Recently, the wafers have become thinner with the improvement of the technique of polishing the wafers 3 and by the request of the users. For example, a standard thickness of a conventional wafer 3 was about 0.6 mm, whereas a recent wafer can be polished to a thickness of about 0.1 mm. As the wafer 3 is thinned, a back circumferential edge portion of the wafer 3 is wedged into the back-around portions 701 of the wafer cassette 4.
The wedged state of the circumferential edge portion of the wafer 3 into the back-around portions 701 poses the following problems.
(1) When the wafer 3 is moved up by the transfer arm 5, in the state where the back edge portion of the wafer 3 is wedged into the back-around portions 701, the wafer 3 is bent by force, resulting in damage of the wafer.
(2) When the back circumferential edge portion of the wafer 3 is wedged into the back-around portions 701, the wafer 3 is frequently stored with a forward or backward inclination. Therefore, it is possible that a space is left between the transfer arm 5 and the wafer 3, resulting in an error in attraction by the attracting portion 6.
(3) The wafer cassette 4 storing wafers 3 is transferred between different steps and factories with the wafer-inlet/outlet side facing up. Therefore, the back circumferential edge portion of the wafer 3 may probably be wedged into the back-around portions 701 due to its own weight.
When the wafers are transferred by a truck or an airplane, they are moved by a transport cassette having a narrower slot width, in consideration of instability during transportation, such as vibration. When they are replaced from the transport cassette to a transfer cassette by the transfer arm or the transport cassette is used as a transfer cassette without replacement, it is highly possible that the back circumferential edge portion of the wafer 3 is wedged into the back-around portions 701 and the wafer 3 is stored with a forward or backward inclination. When the wafer 3 which is not held horizontally is to be transferred by the transfer arm, since the distance between wafers is narrow, the transfer arm 5 easily interferes with the wafer 3. In addition, an attraction error may be caused.
On the other hand, in a case where the wafers are replaced from the transport cassette to a transfer cassette, the engagement must be released manually to replace the wafers, which requires a troublesome process.